This invention relates to an improved process for the continuous thermal cleavage of N-monosubstituted carbamic acid alkyl esters and a process for the preparation of isocyanates which at normal pressure have a boiling point at least 50.degree. C. below that of the isocyanate obtained from the thermal cleavage.
The thermal cleavage of N-monosubstituted carbamic acid alkyl esters has long been known. As demonstrated by the work of A. W. Hoffmann (Berichte der Deutschen Chemischen Gesellschaft, Year 1870, page 653 et seq) and M. Metayer (Bull. Soc. Chim. France, Year 1951, page 802 et seq.), these cleavage reactions are reversible, i.e. when the hot reaction mixtures cool, the isocyanates recombine with the alcohols to form carbamic acid esters. Special measures are therefore required if the isocyanates and alcohols obtained from the thermal cleavage of carbamic acid esters are to be recovered separately.
U.S. Pat. No. 2,409,712 describes a process in which the recombination of isocyanates and alcohols after the thermal cleavage of carbamic acid esters is prevented by immediate separation of the cleavage products. Such separation may be accomplished by introduction of the gases of thermolysis into a cyclohexane-water mixture or by rapid distillation. Although this process is suitable for the discontinuous preparation of isocyanates on a laboratory scale, it is not suitable for a commercial process because immediate separation of the cleavage products is extremely difficult from a technical standpoint. Moreover, the process described in this patent provides only moderate yields of isocyanate, as can be seen from the examples given therein.
It is also known that when N-monosubstituted carbamic acid esters are subjected to heat, they may undergo partial or complete irreversible decomposition. As the investigations of H. Schiff (Berichte der Deutschen Chemischen Gesellschaft, Year 1870, page 649 et seq.) and of E. Dyer and G. C. Wright (J. Amer. Chem. Soc. Volume 81, Year 1959, page 2138 et seq.) have shown the decomposition products may include substituted ureas, biurets, carbodiimides, isocyanurates, secondary amines, olefines and/or carbon dioxide. These decomposition reactions not only reduce the isocyanate yield but may also interfere with processing equipment. For example, difficulty soluble ureas or isocyanaurates may cause blockages in pipes. Carbon dioxide and gaseous olefins may heavily charge the distillation columns with gas. Lastly, basic materials which form as by-products may catalyze irreversible decomposition reactions of carbamic acid esters.
Various processes have been developed in an effort to suppress the decompositions which accompany thermal cleavage. One approach is to reduce the amount of heat used in the cleavage reaction. Such processes are, however, disadvantageous in that the thermal cleavage must generally be carried out in the presence of a catalyst since the volume/time yields would otherwise be too low. In any event, the cleavage of carbamic acid esters into isocyanates and alcohols is by its nature a process in which the application of at least a minimum amount of heat is unavoidable, whether catalysts are used or not.
Processes for the preparation of isocyanates by thermal cleavage of carbamic acid esters in the presence of basic catalysts have been described in U.S. Pat. Nos. 2,713,591; 2,692,275 and 2,727,020 and in Japanese Patent Application No. 54-88201 (1979). Use of basic catalysts may, however, lead to increased irreversible decomposition reactions of carbamic acid esters. (See e.g., J. Appl. Polym. Sci., Volume 16, Year 1972, page 1213). Processes using basic catalysts can therefore result in acceptable isocyanate yields only if the carbamic acid esters used are protected against decomposition by means of suitable substituents.
Another possible method for suppressing side reactions in the thermal cleavage of carbamic acid esters is dilution of the carbamic acid esters and/or the gases of thermolysis with inert diluents. In the processes described in U.S. Pat. No. 3,919,279, German Offenlegungsschrift No. 2,635,490 and Japanese Patent Applications 54-39002 (1979) and 54-88222 (1979), thermal cleavage of carbamic acid esters is carried out in inert solvents, optionally in the presence of certain catalysts. In the processes described in German Auslegeschriften Nos. 2,421,503 and 2,526,193, carrier gases are used in addition to inert solvents, optionally in the form of vaporized low boiling solvents.
The use of solvents in the thermal cleavage of carbamic acid esters does, however, present serious difficulties. The solvent must be stable under the conditions of thermolysis and it must also be inert with respect to isocyanates. The solvent must also be readily miscible with carbamic acid esters and have a vapor pressure at the temperatures employed low enough that it will remain substantially in the liquid phase during thermolysis. These requirements severely limit the choice of solvents. Suitable inexpensive solvents are difficult to find, particularly for the cleavage of carbamic acid esters which have a high molecular weight. Moreover, the use of solvents reduces the volume/time yields of isocyanates. Yet another disadvantage is that when high boiling solvents are used, it is difficult to separate the pure components (residues of isocyanate and carbamic acid ester, and solvent) from the residue in the liquid reaction mixtures by distillation. (See e.g. German Auslegeschrift No. 2,530,001). Further, the working-up and storing of inert diluents entails considerable additional capital expenditure.
U.S. Pat. Nos. 3,734,941 and 3,870,739 describe processes in which carbamic acid esters are split at high temperatures (400.degree. to 600.degree. C. and 350.degree. to 550.degree. C.) in the gaseous phase. One disadvantage of such a process is that the dwell times of the gases in the high temperature range must be short to avoid extensive decomposition of the carbamic acid esters and/or the isocyanates formed due to the high temperature which would otherwise occur in spite of the dilution by the gaseous phase. Short dwell times, however, result in correspondingly low yields of isocyanates. Moreover, this process entails considerable technical difficulty since gases are difficult to heat and cool within a short time due to their low thermal conductivity.